Since I have osteoporosis and can no longer jog, I've been looking for low- or no-impact activities.  I understand swimming does not improve bone density.
D015519:Bone Density / Q000502:physiology; D005081:Exercise Therapy; D005260:Female; D006801:Humans; D008875:Middle Aged; D010024:Osteoporosis / Q000628:therapy*; D013550:Swimming; D016138:Walking / Q000502:physiology*